Continuous digestion process



l Feb/[8, 1938. c. o. TURNER Er AL CONTINUOUS DIGESTION PROCESS Filed Feb. l, 1935 Planear-gb. s,

l l I 2,107,919

Helvlnlowcllwm stipula,

Application February 1, ms, sci-uu N0. 4.552

This invention relates to the extraction of a solid with a solvent in a process of continuous pressure digestion. It relates particularly to, and has as its chief object, the provision of an improved method of continuously extracting theA alumina from alumina-bearing materials by pressure digestion with caustic liquor, and an improved system adapted to carry out such a process.

. .,'I'he'copending application of Weigel,v Serial No. 561,081, discloses a continuous method of digesting bauxite with caustic liquor. 'I'his continuous processl consists broadly in heating the mixture of bauxite and caustic liquor insuccessive stages'of increasing temperature and pressure 20. process, steam released in the cooling stages is in- Jected in some of the preliminary heating stages, thus materially increasing the utilization Aof the heat supplied to the system. Difference in pressure throughout the system is secured as'arule lrvdrostatically by positioning -the component parts of the system in a plurality of dierent elevations. 'Ihis invention allowed materially increased thermal and material eillciencies to be secured and was a great advance over the older batch system.

The present invention is similar to the abovedescribed process of Weigel in that it is capable of continuous operation through holding and coolingvstages and in that heat released from the cooling stages may be utilized in the digestion.

Our invention contemplates an essentially simpliiled process with provision for carrying it out in a system constructed substantially on one level with few moving parts and requiring a minimum of capital expense and operating labor. Provision is made for emciently heating the digestion with steam without direct contact so that dilution due to Vthe condensation of the steam is avoided. Owing to the elimination of the direct injection of steam as a heating means it has been found possible to usefully employ very low pressure steam for heating, as there is no direct mechanical relationship between the pressure in the steam system 4and the pressure in the digesting system. This gives a greater exibillty of control than would otherwise be possible. With independent control over the temperatures and pressures and working with a substantially uniform flow of undiluted digest mixture it has been found practical to make a separation of the caustic digest liquor from the solid residue after dlscstion at an elevated temperature equal to or greater than the normal boiling point of the liquor under atmospheric pressure, as will be further described hereinbelow. o It has long been a desideratum to apply steam heat indirectly to a batch pressure digestion to avoid the dilution arising from heating by direct injection of steam.. This has proved diillcult, as the digestion mixture precipitates solidson the 10 heated surfaces, which seriously interfere with the transfer of heat. In a batch process this dimculty may be overcome to a certain extent by employing high pressure steam and carefully cleaning the heating surfaces between batches. l5 In a continuous .heat regenerative digestion process this problem has presented a serious dimcuity because. no' opportunity is afforded for cleaning the heating surfaces and a layer of solid material of increasing thickness is continually formed. Furthermore, lin a heat regenerative system the dierence in temperature between the material to be heated and the heating medium is small, and a material resistance to heat ilow'or transfer between-the two substantially negatives 25 the advantages of such a regenerative process. v In our solution of-this problem it was found that the caustic liquor, which at times contains small amounts of alumina and other substances in solution from prior treatments, may be satisfactorily superheated in respect to the desired digestion temperatures in known types of heaters, such as a straight tube heat exchanger, without the formation of scale on the heater` walls. vIt was also found that a substantially complete extraction of all available alumina from a1umina-bearing material might be made by treating such material with superheated caustic liquor to produce a digestion mixture at a'predetermined digestion temperature. In actual practice it has been 40 found desirable to wet the alumina-bearing material with sufllcient caustic liquor to form a thick slurry which can be conveniently introduced into the digestion process under pressure, as by pumping.

In the preferred practice of our invention the caustic liquor is divided. This liquor may be at room temperature or at a somewhat elevated temperature, depending upon the general operation -of which this process may form a part. One portion is passed under pressure through 'a step of indirect heating, which may consist of several stages of increasing temperature until the liquor has been given a superheat with respect to the contemplated digestion temperature. The other portion, preferably the smaller portion, is added to the alumina-bearing material to form a ,thick slurry which is subsequently maintained under pressure. -The two portions are then combined under pressure and the superheat of the liquor portion is absorbed by the slurry portion to produce a digestion mixture having a temperature predetermined by the degree of lsuperheat in the liquor. The digestion mixture'is passed through a plurality of holding stages under these conditions of `temperature and pressure until substantially all of the available alumina is dissolved by the caustic liquor. Under kthese conditions the digest solution is rendered stable and there is no precipitation of alumina, which assures a. maximum yield. The mixture is then passed through` a plurality of cooling stages. Throughout the entire cycle of operation a constant flow of digestion mixture from step to step and from stage to stage is preferably maintained. During each cooling stage heat is absorbed in the evaporation of water/from the liquor under reduced pressure. This heat may be applied to the liquor-heating stages and, owing to the use of indirect heating, steam generated atsubstantially any pressure in the cooling stages may be efficiently utilized in supplying heatto the heating steps. Even very low pressure exhaust steam may be used 4in the initial stages of liquor heating. i

For the purpose of explanation, reference will be had to the drawing which forms a part of this specification and which represents, in schematic outline, a flow sheet of a process vembodying .this invention in the extraction of alumina from alumina-bearing material.

Referring. to the drawing, the apparatus which may be used in the practice of our invention comprises proportioning feed hopper I for the alumina-bearing material and caustic liquor proportioning tank 2, slurry mixer 4, surge tank 6, tubular liquor heaters I4, I5, I6 and I1, digesters II, 2I, 22, 23, 24 and 25, cooling chambers 26, 21 and 28, thickener 44, and tank-connecting and mixture-transferring means hereinafter referred to. The slurry mixer andthe digesters are equipped with revolving stirring arms 52 to prevent the solids settling from the liquor. It will be understood that while the process will be described as having specifically four heating stages and three cooling stages and six digesting vessels, this is merely by way of example, and fewer or more stages may be used to best satisfy any particular condition.

The reagents, that is, alumina-bearing mate- Irial, which may be bauxite, and caustic liquor,

by which is meant solutions 'of potassium or l sodium hydroxide which may contain in addition small amounts of other substances, such as carbonates and aluminates, are drawn continuously at a constant rate or in batches from the tanks I and 2 respectively. The bauxite passes through the proportioning apparatus 3 directly into a slurry mixer 4. The caustic liquor passes through the pipe line 5 which has two branches, onelleading to the slurry mixer 4 and the other to the clear liquor surge tank 6. The two branch lines are equipped with proportioning orificesr 1 and 8 which permit a measured flow of both tank s through the une la to the heater' aimais I4 which may be a tubular heater, as indicated, through which it passes and is partially brought to a superheated condition by heat supplied by low pressure steam through the tube walls. In practice it is desirable to use oversize pumps and reduce the flow by valves in the pump discharge line, such as 9a and I2a. From the heater I4 the liquor passes through similar heaters I5 and I6 which are heated progressively by somewhat higher pressures of steam to higher temperatures, after whichA the liquor finally passes through a heater 'I1 which is heated with still higher pressure steam, which lraises the temperature of the liquor to a superheat with respect to the other temperatures employed in the process.

The superheated liquor from l1 passes through.

pipe line I8 into the digester II. The superheat of the liquor is given up to the slurryl from mixer 4 and may be adjusted to give the desired diges l line 20.

From the digestion vessel I I the mixture vpasses through similar digesters 2|, 22, 23, 24 and 25 under substantially the conditions of temperature and pressure maintained in digester II. During this passage the alumina is in a large part extracted in digesters II and 2|. Digesters 2|, 22, 23, 24 and 25 serve aslholding digesters to allow sufficient time to elapse to substantially complete the digestion or solution of all of the available alumina andl to stabilize the digestion mixture. v

The digestion mixture, which now consists of a caustic aluminate solution and the insoluble residue from the bauxite, passes from digester 25 to the cooling tanks or blow-off tanks 26, 21 and 28. The digestion mixture is released continually from digester 25 through the pipe line 29 and the control or blow-off valve 30 to tank 26. 'Ihe pressure'inV tank 26 is lower than the digestion pressure and steam isv released. The steam passes through pipe 3I'and control valve 32 to the tubular heater I6. The pressure in tank 26 is largely controlled or set by valve 32 and is higher than the pressure in tank, 21. The mixture therefore flows from tank 26 to tank 21 through the loop pipe seal 33 and the valve 34. Steam is again released at lower pressures and passes through the pipe line 35 and valve 36 to heater I5. The cooling mixture is .still under substantial pressure and ows through loop seal pipe line 31 to tank 28. Steam is released in this tank and is led to heater I4 through the pipe 38 and the control valve 39. The mixture is drawn from tank 28 by a pump 48 through pipe 4I and delivered to lters for separating the residue from the liquor, not shown, which is a step preliminary to treating the caustic liquor for the recovery of alumina therefrom in known manner, which treatment forms no part` of this invention.

In this process the mixture of bauxite and liquor is digested without dilutionfrom the condensate of heating steamand,V in fact, a certain not unsubstantial amount of concentration is effected by the amount of steam released in the blow-off tanks. It is sometimes desirable, for instance, in order to reduce capital: expense, to heat the mixture of caustic liquor from the regenerative heaters I4, I5 and I6 and slurry from mixer 4 by the direct injection of high pressure steam in the digester II` and thereby eliminate the 'superheater I1. In this case the digestion temperature is controlled by the thermostatic valve 42 in the steam supply line 26. In this modication of our invention all of the advantages of the various steps of heat regeneration are preserved unaffected, and because of the fact that the digestion mixture is partially heated indirectly and to a degree concentrated, satisfactoryoperation may be secured by the injection of high pressure steam to effect the last stage ofheating. If desired, a combination of both means of heating may be used. 1

Primarily,l the flow of materials through the process is controlled by the pumps 9 and I2 and valves 9a and I2a, which maintain a pressure at least Aequal to the pressure in the digesters, and the blow-off valve 3II. Secondarily, the ow is `controlled by the valves 32, 36 and 39 and the pumplw. `If desired, the elevation of the blow-off tanks 26, 21 and 23 may be adjusted to secure the desired action through the loop seals 33 and 31. The temperature of the process is controlled primarily by valves I9 and 42 which regulate the primary source of heat; Secondarily, the temperature is controlled through the valves 32, 36 and 39 and the orifices 1 and 3.

They ow of materials is continuous throughout are kept filled with the process. The digesters material at all times by the pumps 9 andl I2 and the blow-off valve 30. The range of possible adjustmentsof valves 32, 36 and 39 is very wide, as the steam entering the heaters I4, I5 vand I6 has no mechanical relation to the pressure maintained on the digestion solution, and the heat can therefore be supplied by steam at any desired pressure having the desired temperature within the range of the pressure of the released steam. This gives the utmost flexibility in controlling the conditions of material flow, temperatur'e and pressure.

In such a process having a continuous flow of materials at predetermined controllable pressures and temperatures as described, it has been found practical to insert a step of thickening or separation of the solid residue dispersed in the caustic aluminate liquor after the digestion is complete. The residue may be, therefore, in this case, discharged separately from the process as a thick sludge containing a small amount of liquor and the extract liquor discharged substantially clear an'd containing very little residue. The technical prerequisites for such a separation are that the ow of material through the thickener be constant, or'nearly so, and that the temperature and the c'oncentration of dissolved material be such that no precipitation occurs in 1. the thickener. The practical prerequisite is that in addition the amount (i. e., the volume) to be treated in the thickener be small, and`v that the settling rate be rapid; otherwise, the size of thickener necessary may become larger than can be economically operated under the conditions of.

temperature and pressure, or the time required may be so long as to negative any advantage in its use. In the case of vextracting alumina with caustic liquor as described, the.thickening step may .be placed, for example, Vbetween the rst two stages of cooling, that is, between blowolf tanks 26 and 21. This is shown in the drawing. The enclosed thickener 44 may be connected in the system in the-loop seal pipe line 33 by closing valve 34 and opening valves 45, 46 and 41. This -mixture discharged from tank 26 ows through valve 45 andV pipe 43 into the thickener below the liquid level where the residue settles in the cone bottom and is drawn oil.' as a sludgeA ucr overflows into the between said vapor 49 by any suitable not shown. The clear liqtank 50 and flows through the valve 46 into the loop seal pipe line 33 to tank 21. The pipes 5I equalize the pressure throughout the thickening steps and maintain a pressure substantially equal to the pressure of tank 26, and the thickener is of course thus maintained under approximately the temperature of tank 26. The conditions of thickening above set forth have been found to materially increase the settling rate of ,the solid material from the mixture, which serves to decrease the size of the thickener or settling equipment. For example, the settling rate may be increased fty per cent or more by elevating the tempera-ture. 'I'he temperature is preferably increased mal boiling point of the liquid at atmospheric pressure.' This remarkable change in settling rate is thought to be due principally to a change through the discharge pipe form of sludge pump,

above the norin viscosity of the liquor with temperature, but

it is not desired to limit the fact this explanation.

It will be noted that in the operation of this process now being described it is necessary only to apply heat at one point, heater I1 or digester` II, which is done with steam pressurel as from the steam supply line 23, which in turn may be connected to any source of steam oi' sumcient pressure.

Although we have described our invention with reference to .a particular embodiment thereof, it will be apparent that it is by no means limited to the details of operation, temperature, pressure, composition or substances described` but may be utilized to advantage in any and all similar methods of pressure digesting aluminous material or other solids with a liquid.

We claim:

1. A method of continuously pressure digesting solids with a solvent, comprising forming a slurry of the solids with some of the solvent, and

in any way by A vply heat to several. of the solvent-heating stages.

the heating of the solvent in several such stages being accomplished by passing said released vaporinto heat exchangers which prevent contact and said solvent, and maintaining a flow of materialsthroughout all the steps of the process.

2. A method of continuously extracting alumina from aluminous material, which comprises forming a slurry of said aluminous material and caustic liquor, indirectly heating `a further amount of caustic liquor in a plurality of successive lseparate stages of increasing temperatures to a temperaturey such that when combined with the slurry a predetermined temperature of the mixture is attained, combining the slurry and the heated caustic liquor under pressure, and digesting under suitable conditions of temperature and pressure until of the alumina is in solution, then cooling the mixture in a plurality of separate stages of desubstantially all creasing temperature and pressure, predetermining the decrease in pressure between successive cooling stages so as to release steam from the caustic liquor in such quantity that it is adapted to supply heat for several of the caustic liquor heating stages by passing said released steam into heat exchangers which prevent contact between said steam and said caustic liquor, and maintaining a ow of materials throughout all the steps of the process.

3. A method of continuously extracting alumina from. aluminous material, which comprises forming a slurry of said aluminous material and caustic liquor, indirectly heating a further amount of caustic liquor in a plurality of successive separate stages of increasing temperature to a suiilcient temperature so that when combined with the slurry a predetermined temperature of the mixture is attained, combining the slurry fand the heated caustic liquor and digesting under suitable conditions. of temperature and pressure until substantially all of the alumina is in solution, then cooling the mixture in a plurality of separate stages of decreasing temperature and pressure, the decrease in pressure between successive cooling stages being such that steam is released from the caustic liquor in such quantity that it is adapted to supply heat for' several of the caustic liquor heating stages, the

heating of the caustic liquor in several such stages being accomplished 'by passing said released steam and caustic liquor into heat exchangers which prevent contact between said steam and said caustic liquor, separating by sedimentation the undissolved portion from the liquor solution at a temperature higher than the boiling point of said liquor at atmospheric pressure and maintaining a ilow of materials throughout all the steps of the process.

4. Apparatus for extracting alumina from alug minous material, which comprises, in combination, means for forming a slurry of aluminous material and caustic liquor, means for indirectly heating caustic liquor in a plurality of separate stages of increasing temperature, means for combining and digesting the slurry and caustic liquor under controlled conditions of temperature and pressure, means for cooling the digestion product in a plurality of successive stages of decreasing temperatures and pressures, means for indirect heat exchange between said heating and cooling means, and means for maintaining a constant ilow of material throughout.

5. Apparatus for extracting alumina from aluminous material, which comprises, in combination, means for forming a slurry of aluminous material and caustic liquor, means for indirectlyheating caustic liquor in a plurality of separate liquor, which comprises dividing the' caustic liquor into major and minor portions, forming a yslurry of the aluminous material with the minor portion and heating the major portion indirectly bining said slurry and heated caustic liquor under pressure, digesting the mixture under controlled conditions of temperature and pressure until substantially all of the alumina is in solution, and

maintaining a ilow of materials throughout all the steps of the process.

'7. vIn apparatus for extracting solids with solvents by continuous pressure digestion, the combination comprising a mixing unit adapted to form a pumpable slurry from a solid to be extracted and a solvent, a plurality of digesting tanks, means for continuously supplying said slurry tov said digesting tanks, means forl continuously supplying additional solvent to said digesting tanks in an amount having a predetermined ratio to the amount supplied to said mixing unit,

a plurality of heat exchange units adapted to raise the temperature of said additional solvent in progressive steps, a plurality of cooling chambers connected in series, transfer means adapted to deliver solution and undissolved solids from said digesting tanks to said 'cooling chambers, means for regulating the pressure in the digesting tanks, control means between adjacent cooling chambers in said series adapted to maintain progressively decreasing pressures in said chambers, and means for conveying the vapor released in atleast Ione of said cooling chambers to at least one of said heat exchange units.

8. In apparatus for extracting solids with solvents by continuous pressure digestion, the combination comprising a surge tank for solvent, a mixing unit adapted to form a pumpable slurry from a solid to be extracted and a solvent, means for supplying said solid to said mixing unit, means for supplying said solvent to said mixing unit, means for supplying solvent to said surge tank, a plurality of digesting tanks connected in series, means for continuously supplying said slurry to the rst of said series of digesting tanks from said mixingunit, means for continuously supplying additional solvent to the first of said series of digesting tanks from said surge tank in an amount- Ihaving a predetermined ratio to the amount supplied to said mixing unit, a plurality of heat exchange unitsinterposed in the supply means for additional solvent and adapted to raise the temperature of said additional solvent in progressive steps, a plurality of cooling chambers connected in series, transfer means adapted to deliver solutionand undissolved solids from the last of said series of digesting tanks to the irst of said series of cooling chambers and including means for regulating pressure in the digesting tanks, control means between adjacent cooling chambers in said series adapted to maintain pressures in said chambers decreasing progressively in steps from the digestion pressure, and means for conveying vapor released by the decrease of pressure in at least one of said cooling chambers to at least one of said heat exchange units.

9. In apparatus for extracting solidsA with solvents by continuous pressure digestion, the combination comprising a mixing unit adapted to form a pumpable slurry from a solid to be extracted and a solvent, a plurality of digesting tanks, means for continuously supplying slurry to said'digesting tanks from said mixing unit, means for continuously supplying additional solvent to said digesting tanks, a plurality of cooling chambers connected in series, transfer means 'ItA cent cooling chambers in said series and adapted to separate solids from solution under substantially the conditions of superatmospheric pressure and elevated temperature maintained in the ilrst of said connected cooling chambers.

10. In apparatus for extracting solids with a solvent by continuous pressure digestion, the

combination comprising a mixing tank adapted vto Iorm a slurry of solid and solvent, at least one heat exchange unit, at least one digesting unit,

at least one cooling chamber, controlled means for delivering slurry from said mixing tank to- 'said digesting unit, means for passing a controlled quantity of ,additional solvent through saidv heat exchange unit and into said digesting unit at a controlled temperature and pressure, and means for delivering material from said digesting unit to said cooling chamber while maintaining-the desiredvpressure in the digesting unit.

11. The method of extracting alumina from aluminous material with caustic liquor, comprising mixing part of the caustic liquor with aluminous material to form a slurry, heating a further ing mixing solid aluminous material with caustic liquor to form a slurry, `heating a further quantity oi! caustic liquor by indirect contact with steam, digesting said slurry and heated caustic liquor in a closed vessel maintained at the desired superatmospheric pressure and elevated temperature by direct contact with steam to dissolve alumina, reducing the pressure of the resulting solution and withdrawing steam liberated thereby, and returning the steam to indirectly heat'a further quantity of caustic liquor.

13. I'he method of extracting alumina from aluminous material, comprising digesting aluminous material' with preheated caustic liquor in a closed vessel, maintaining the pressure above atmospheric and the temperature above the boiling l point at iatmospheric pressure by injecting steam into said vessel, separating the resulting solution from undissolved residues by sedimentation at elevated temperature and superatmospheric pressure, reducing the pressure on said solution by withdrawing vapor liberated thereby, and employing said vapor to preheat a further quantity of said caustic liquor.4

` 14. The methodvof extracting alumina from aluminous material, comprising digesting aluminous material with caustic liquor atsuperatmospheric pressure and at a temperature higher than the boiling point of the liquor at atmospheric pressure, land. separating the resulting solution from undissolved residues by sedimentation at elevated temperature and superatmospheric pressure.

15. The method of extracting alumina from aluminous material, comprising digesting alum!- nous material with caustic liquor at superat mcspheric pressure and at a temperature higher than the boiling point of the liquor at atmospheric pressure, and maintainingv the resulting solution and undissolved residues at superatmospheric pressure and elevated temperature until said residues are separated'from the solutionby sedimentation.

CARRELL ORISON TURNER. MELVIN POWELL WEIGEL. 

